Artificial Disc Replacement Device

ABSTRACT

An artificial disc replacement device is disclosed. The device includes an upper endplate and a lower endplate, as well as a core assembly disposed between the endplates. The core assembly includes a core member with a curved engaging surface and a matrix member. The matrix member is more compressible than the core member. The curved engaging surface of the core member engages a recess in the upper endplate so that the upper endplate can translate along the curved engaging surface. The curved engaging surface has a greater curvature at its posterior end than at its anterior end to facilitate different ranges of motion during extension and flexion.

BACKGROUND

The embodiments are generally directed to implants for the spine, and inparticular to artificial disc replacements.

An artificial disc, also known as artificial disc replacement (ADR), isa medical device implanted into the spine that acts as, or imitates, aspinal disc. Surgeons can replace the entire disc or remove only thenucleus (center of the disc). ADR devices may be designed to allow formotion between the adjacent vertebra, rather than fuse adjacentvertebrae together as occurs in other kinds of spinal implants.

Current ADR systems may impede normal movement in the back and/or neck.For example, some systems allow vertebrae to pivot around a fixedcentral point but prevent the vertebrae from translating during flexion.Other ADR systems include a mobile core that can slide along one or bothendplates. These mobile cores may become displaced from their intendedrange of motion, or their range of motion may be affected by scarring ofthe adjacent tissue, which may result in failure of the ADR and/or alimited range of motion. Also, in current ADR systems, the upper andlower endplates may come into contact during flexion and/or extension,thereby requiring the use of high strength materials such as metals. Theuse of metallic endplates may create unwanted visual artifacts duringmagnetic resonance imaging (MRI) of the patients spine. These MRIartifacts may make it difficult for a radiologist and/or surgeon toaccurately assess the state of the spine or surrounding tissue.

There is a need in the art for a system and method that addresses theshortcomings discussed above.

SUMMARY

In one aspect, an artificial disc replacement device includes a firstendplate configured to be disposed against a first vertebrae in a spineand a second endplate configured to be disposed against a secondvertebrae in the spine. The device also includes a core assemblycomprising a core member and a matrix member, where the core memberincludes a base portion and a curved engaging surface, and where thebase portion of the core member is embedded in the matrix member. Thefirst endplate includes a first recess for receiving the curved engagingportion of the core member and the second endplate includes a secondrecess for receiving the matrix member.

In another aspect, an artificial disc replacement device includes afirst endplate configured to be disposed against a first vertebrae in aspine and a second endplate configured to be disposed against a secondvertebrae in the spine. The device also includes a core member disposedbetween the first endplate and the second endplate, the core memberhaving a curved engaging surface for engaging the first endplate. Thecore member includes a sagittal plane that separates the core memberinto a first lateral side and a second lateral side. The core memberalso includes a coronal plane that separates the core member into ananterior side and a posterior side. The curved engaging surface includesa curved boundary that extends within a first plane that is parallelwith the sagittal plane. The curved boundary has a first arc radiusalong an anterior portion disposed on the anterior side of the coremember. The curved boundary has a second arc radius along a posteriorportion disposed on the posterior side of the curved boundary, where thesecond arc radius is substantially different from the first arc radius.

In another aspect, an artificial disc replacement device includes afirst endplate configured to be disposed against a first vertebrae in aspine and a second endplate configured to be disposed against a secondvertebrae in the spine. The device also includes a core member disposedbetween the first endplate and the second endplate and a matrix memberdisposed between the first endplate and the second endplate, where thematrix member is in continuous contact with the second endplate. Thefirst endplate includes a recess and where the core member includes acurved engaging surface in continuous contact with the recess. The firstendplate can move along the curved engaging surface. The matrix memberis disposed between the core member and the second endplate and preventscontact between the core member and the second endplate. The matrixmember is positioned to prevent the first endplate from contacting thesecond endplate as the first endplate moves along the core member.

Other systems, methods, features and advantages of the embodiments willbe, or will become, apparent to one of ordinary skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description and this summary, bewithin the scope of the embodiments, and be protected by the followingclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments can be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, with emphasis instead being placed uponillustrating the principles of the embodiments. Moreover, in thefigures, like reference numerals designate corresponding partsthroughout the different views.

FIG. 1 is a schematic view of a portion of the cervical spine includingan artificial disc replacement device disposed between two adjacentcervical vertebrae, according to an embodiment;

FIG. 2 is a schematic cross-sectional view of the artificial discreplacement device of FIG. 1 including the two adjacent vertebrae;

FIG. 3 is a schematic perspective view of the artificial discreplacement device of FIG. 1;

FIG. 4 is a schematic exploded view of the artificial disc replacementdevice of FIG. 3;

FIG. 5 is a schematic front view of a core assembly including a coremember and a matrix member, according to an embodiment;

FIG. 6 is a schematic front view of the core assembly of FIG. 5, inwhich the matrix member deforms under compressive forces;

FIG. 7 is a side schematic view of a core member of an artificial discreplacement device in which the variable curvature of the upwardengaging surface in the longitudinal direction is visible, according toan embodiment;

FIG. 8 is a schematic front view of the core member of FIG. 7 in whichthe constant curvature of the upward engaging surface in the lateraldirection is visible;

FIG. 9 is a schematic top-down view of an embodiment of a core memberhaving a variable width;

FIG. 10 is a schematic side view of a patient looking forward with anenlarged view of an artificial disc replacement device in a neutralposition, according to an embodiment;

FIG. 11 is a schematic view of the patient of FIG. 10 in which theirneck is undergoing extension with an enlarged view of a correspondingconfiguration of the artificial disc replacement device;

FIG. 12 is a schematic view of the patient of FIG. 10 in which theirneck is undergoing flexion with an enlarged view of a correspondingconfiguration of the artificial disc replacement device;

FIG. 13 is a schematic view of the patient of FIG. 10 in which theirneck is undergoing further flexion with an enlarged view of acorresponding configuration of the artificial disc replacement device;

FIG. 14 is a schematic front view of a patient looking forward with anenlarged view of an artificial disc replacement device in a neutralposition, according to an embodiment;

FIG. 15 is a schematic view of the patient of FIG. 14 in which theirneck is undergoing lateral bending with an enlarged view of acorresponding configuration of the artificial disc replacement device;

FIG. 16 is a schematic view of an endplate of an artificial discreplacement device with a centrally located dome for engaging a recessin an adjacent vertebra, according to an embodiment;

FIG. 17 is a schematic view of an endplate of an artificial discreplacement device with a posteriorly located dome for engaging a recessin an adjacent vertebra, according to an embodiment;

FIG. 18 is a schematic view of an endplate of an artificial discreplacement device with a relatively flat dome for engaging acorresponding flat recess in an adjacent vertebra, according to anembodiment; and

FIG. 19 is a schematic view of another embodiment of a core assembly anda lower endplate, in which a matrix member of the core assembly coversan entirety of the lower endplate.

DETAILED DESCRIPTION

The embodiments provide a spinal implant in the form of an artificialdisc replacement device, also referred to simply as an ADR device. TheADR device can be implanted between adjacent vertebrae in the spine inorder to replace damaged spinal discs. The exemplary ADR device isconfigured with an upper endplate and a lower endplate that sandwich acore assembly. The core assembly includes a core member that ispartially embedded within a matrix member. The core member has a convexupward engaging surface. The upward engaging surface varies in curvaturealong a sagittal plane to facilitate natural motion between the adjacentvertebrae during flexion and extension. Specifically, the posteriorportion of the core member has a smaller arc radius (andcorrespondingly, a larger curvature) than the anterior portion of thecore member. This helps limit compression during extension and increasescompression and forward translation of the upper endplate duringflexion. The upward engaging surface has an approximately constantcurvature along a coronal plane to facilitate symmetric lateral bendingin either lateral directions.

The exemplary device also uses a matrix member that is less rigid thanthe core member, thereby improving cushioning and shock absorption. Thematrix member may also extend upward from the lower endplate to engagethe upper endplate during flexion and extension, thereby preventing theendplates from contacting one another. As the endplates never contactone another they can be constructed from materials other than metal. Forexample, the endplates may be constructed of ceramic or carbon materialsthat provide better biocompatibility with vertebra, improved wearcharacteristics and different degrees of strength compared to metalmaterials. Additionally, using ceramic or carbon materials may limit oreliminate MRI artifacts that can be caused by using metallic endplates.

FIG. 1 is a schematic view of an embodiment of an implant in the form ofan artificial disc replacement device 100, also referred to simply asADR device 100. In FIG. 1, ADR device 100 is embedded within a portion102 of a spinal column. Specifically, ADR device 100 is disposed betweena first vertebra 110 and a second vertebra 112.

It may be appreciated that the ADR device described in this detaileddescription and in the claims may be used within any suitable portion ofthe spine. The embodiment of FIG. 1 depicts ADR device 100 used withlumbar vertebrae. However, other embodiments could include a device thatmay be used to replace spinal discs in any portion of the spineincluding the lumbar spine, the thoracic spine, and the cervical spine.For example, the embodiments described below and shown in FIGS. 10-15depict an ADR device that is implanted between adjacent cervicalvertebrae in the neck. It may be appreciated that suitable adjustmentscould be made to the dimensions and/or geometries of various parts ofthe ADR device in order to accommodate anatomic differences in thedifferent regions of the spine.

For purposes of clarity, reference is made to various directionaladjectives throughout the detailed description and in the claims. Asused herein, the term “anterior” refers to a side or portion of animplant that is intended to be oriented towards, or disposed closer to,the front of the human body when the implant has been placed in thebody. Likewise, the term “posterior” refers to a side or portion of animplant that is intended to be oriented towards, or disposed closer to,the back of the human body following implantation. Moreover, theposterior side and the anterior side of a body (or part) may beseparated by a coronal plane (also known as a frontal plane).

In addition, the term “superior” refers to a side or portion of animplant that is intended to be oriented towards a top (e.g., the head)of the body while “inferior” refers to a side or portion of an implantthat is intended to be oriented towards a bottom of the body. Moreover,the superior side and inferior side of the body may be separated by atransverse plane.

Reference is also made herein to “lateral” sides or portions of animplant, which are sides or portions facing along a lateral direction ofthe body. These lateral sides may be separated by a sagittal plane.

FIG. 2 is a schematic cross-sectional view of ADR device 100 as it isembedded within portion 102 of the spinal column. ADR device 100 is alsoshown in isolation in FIG. 3. Additionally, FIG. 4 depicts an explodedisometric view of ADR device 100.

Referring to FIGS. 2-4, ADR device 100 includes a first endplate 220 anda second endplate 230. First endplate 220, which may also be referred toas an upper endplate, is disposed on a superior side of ADR device 100.Second endplate 230, which may also be referred to as a lower endplate,is disposed on an inferior side of ADR device 100.

First endplate 220 may be configured to engage a first vertebra (forexample, first vertebra 110 of FIG. 2). In some embodiments, firstendplate 220 may include a dome 222 that can be fit into a correspondingrecess (or concave area) within first vertebra 210. Dome 222 comprises aprotrusion that helps align and hold first endplate 220 in position withrespect to first vertebra 110. In particular, dome 222 may help keepfirst endplate 220 from translating laterally with respect to firstvertebra 110.

Second endplate 230 may be configured to engage a second vertebra (forexample, second vertebra 112 of FIG. 2). In some embodiments, secondendplate 230 may include provisions for permanently attaching to avertebra. In the exemplary embodiment, second endplate 230 includes aset of teeth 232 that can engage a vertebra.

Although the embodiments depict endplates with domes and/or teeth forengaging vertebrae, in other embodiments any other suitable provisionsfor attaching endplates to vertebrae could be used. Exemplary mechanismsinclude, but are not limited to: screws, rods, nails, and blades. Insuch embodiments, the endplates may be suitably modified to receive atleast part of the fastening mechanism. For example, an endplate couldinclude holes for receiving screws.

ADR device 100 may also include a core assembly 250 that is disposedbetween first endplate 220 and second endplate 230. Core assembly 250 ofADR device 100 may itself comprise two distinct members. Specifically,core assembly 250 comprises a core member 252 and a matrix member 254.Core member 252 further includes a base portion 260 and a curvedengaging surface 262. As shown in FIG. 2, base portion 260 of coremember 252 may be embedded within matrix member 254.

First endplate 220 may include a recess 224 that receives curvedengaging surface 262 of core member 252. As described in further detailbelow, recess 224 may be sized and shaped to receive different portionsof curved engaging surface 262, allowing first endplate 220 to tilt andtranslate along curved engaging surface 262 so as to facilitate naturalmotions between adjacent vertebra during flexion, extension, and lateralbending. In the exemplary embodiment shown in FIG. 2, recess 224 extendsfrom an inferior side of first endplate 220 towards a superior side offirst endplate 220. However, recess 224 does not extend into the portionof first endplate 220 defined by dome 222. In other embodiments,however, recess 224 could extend into the portion of first endplate 220defined by dome 222, which may enable the overall thickness of firstendplate 220 to be further reduced and/or the depth of recess 224 to beincreased.

Second endplate 230 may include a recess 234 that receives matrix member254. Recess 234 may be bounded by an outer wall or lip 235. In someembodiments, recess 234 is sized and dimensioned to tightly fit matrixmember 254 so that, once inserted, matrix member 254 cannot be easilydislodged from recess 234. However, in other embodiments, matrix member254 can be fixed within recess 234 using a suitable fastener and/orbiocompatible adhesive.

Vertebral bodies are comprised of two distinct types of tissue: corticalbone and cancellous bone. The cancellous bone is disposed more centrallywithin the vertebral body, which is surrounded by an outer layer ofcortical bone. At the superior and inferior ends of the vertebral body,the cortical bone forms a raised lip (or cortical rim) around thecancellous bone. Because the rim of cortical bone is denser andgenerally stronger than the interior cancellous bone, the endplates ofthe present embodiments are shaped to increase engagement with thecortical rim. Specifically, first endplate 220 and second endplate 230both have approximately rectangular shapes with rounded corners. Thisgeometry helps ensure that the endplates are supported, at least inpart, by the cortical rim of each vertebral body.

Although the embodiments depict endplates with an approximatelyrectangular geometry, in other embodiments the outer perimeter of eachendplate may be varied to approximately match the geometry of thecortical rim of the associated vertebral body. Thus, in some cases,endplates could have a curved front edge that matches the approximategeometry of the anterior lip of the associated vertebral body. Likewise,in some cases, endplates could have lateral and/or posterior edges thatmatch the approximate geometries of the lateral and/or posteriorportions of the cortical rim.

FIG. 5 is a schematic view of core assembly 250 shown in isolation. Asseen in FIG. 5, base portion 260 may be embedded in matrix member 254.That is, base portion 260 may be surrounded from an inferior (lower)side, as well as from the lateral sides, by portions of matrix member254. However, matrix member 254 does not cover or encase a substantialportion of curved engaging surface 262, which extends out from asuperior surface 502 of matrix member 254. With this configuration,curved engaging surface 262 directly engages first endplate 220 (seeFIG. 2), while preventing any portion of core member 252 from cominginto direct contact with second endplate 230.

Core member 252 and matrix member 254 may be made of substantiallydifferent materials. In some embodiments, core member 252 may be made ofa more rigid material than matrix member 254. Also, matrix member 254may be made of a substantially more compressible material than coremember 252. For example, as seen in FIG. 6, matrix member 254 mayundergo substantial compression in a vertical direction under verticalloads 602. By contrast, core member 252 may not compress substantiallyunder these same loads. As seen by comparing FIGS. 5 and 6, the verticalthickness of matrix member 254 may compress from a first thickness 520to a second thickness 620 while core member 252 maintains asubstantially constant thickness 522 before and during compression.

Embedding part of core member 252 within a more flexible and/orcompressible matrix material allows for cushioning and shock absorptionwithin ADR device 100. Furthermore, this configuration allows coremember 252 to “float” within matrix member 254, thereby providing somerelative movement between core member 252 and second endplate 230 (seeFIG. 2). Moreover, because matrix member 254 separates core member 252and second endplate 230, there are no frictional or other contact forcesgenerated between these two components as they move relative to oneanother. This may reduce the likelihood of mechanical failure. Asdescribed in further detail below, matrix member 254 may also provide acushion that prevents the opposing endplates from coming into contactwith one another during flexion or extension. This eliminates contactforces between the endplates and allows for the use of a wider range ofnon-metallic materials for the endplates. Additionally, by using aflexible and/or compressible matrix member, the present embodimentsfacilitate vertical cushioning during compression. That is, the verticaldisplacement between the upper and lower endplates at one end may bedecreased, in part, by the vertical compression of the matrix member.This facilitates compression of the spine along the anterior side duringflexion while also providing shock absorption.

In different embodiments, a variety of suitable materials could be usedfor the components of an ADR device. Suitable materials for a coremember include, but are not limited to: metallic materials, plasticmaterials, and/or ceramic materials. Suitable materials for a matrixmember include, but are not limited to: plastics and gels. Suitablematerials for endplates include, but are not limited to: metallicmaterials, plastics, and/or ceramics. In one exemplary embodiment, acore member could be comprised of a polyether ether ketone (PEEK)material. Also, in one embodiment, a matrix member could be comprised ofa suitably flexible gel. Additionally, in one embodiment, one or bothendplates could be comprised of a suitable ceramic material.

The embodiments provide an artificial disc replacement that facilitatesnatural motion between vertebra. This is accomplished, in part, by thegeometry of the core member, which has a curvature that varies overdifferent regions. In particular, the curvature may vary between theposterior and anterior sides of the core member. That is, the radius ofcurvature changes from the posterior end to the anterior end.

FIG. 7 is a schematic side view of core member 252. Visible within FIG.7 is a sagittal plane 702 and a coronal plane 704 (shown here as aline). Coronal plane 704 divides core member 252 into a posterior sideand an anterior side. Core member 252 further includes a posteriorportion 710 associated with its posterior side and an anterior portion712 associated with its anterior side.

The posterior and anterior portions may be associated with differentdegrees of curvature. In this description and in the claims, thecurvature of a surface may be characterized by its radius of curvature,or arc radius. The arc radius of an arc, for example, is the radius ofthe circle of which the arc is a part. Moreover, the curvature of alocal portion of a surface is inversely proportional to the associatedarc radius of that local portion. In particular, a larger arc radius isassociated with a smaller degree of curvature, while a smaller arcradius is associated with a larger degree of curvature.

As seen in FIG. 7, curved engaging surface 262 includes a curvedboundary 720 that lies within sagittal plane 702. Curved boundary 720has a curvature that varies along its length. Specifically, alongposterior portion 710, curved boundary 720 has a first arc radius 730.Likewise, along anterior portion 712, curved boundary 720 has a secondarc radius 732. As shown schematically in FIG. 7, first arc radius 730is substantially smaller than second arc radius 732. This corresponds toa greater curvature for posterior portion 710 than for anterior portion712.

In some embodiments, the arc radius of curved boundary 720 may graduallychange between first arc radius 730 to second arc radius 732. This mayoccur within a transition portion 740 that is intermediate to posteriorportion 710 and anterior portion 712. In some cases, the arc radiuscould vary smoothly from first arc radius 730 to second arc radius 732.In other cases, however, the arc radius could abruptly change.

In different embodiments, the relative sizes of each arc radius couldvary. That is, their ratios could vary. In some embodiments, the ratioof the first arc radius to the second arc radius could vary in the rangeapproximately between 1:2 and 9:10. In some embodiments, the ratio ofthe first arc radius to the second arc radius could be approximately3:4. It may be appreciated that the ratio of the arc radii of theposterior and anterior ends may be suitably changed to accommodatedifferent intended ranges of motion in different portions of the spine.For example, the ratio could have one value for implants used in thecervical spine and another ratio for implants used in the lumbar spine.

Additionally, the absolute sizes of each arc radius could vary indifferent embodiments. In some embodiments, the first arc radius couldhave a value approximately in the range between 4 and 6 centimeters. Inone embodiment, the first arc radius could have a value of approximately4.5 centimeters. In some embodiments, the second arc radius could have avalue approximately in the range between 5 and 7 centimeters. In oneembodiment, the first arc radius could have a value of approximately 6centimeters. It may be appreciated that the absolute values of the arcradii of the posterior and anterior ends may be suitably changed toaccommodate different intended ranges of motion in different portions ofthe spine. For example, the arc radii could have one set of value forimplants used in the cervical spine and another set of values forimplants used in the lumbar spine.

FIG. 8 is a schematic cross-sectional front view of core member 252.Visible within FIG. 8 is sagittal plane 702 (shown here as a line) andcoronal plane 704. Sagittal plane 702 divides core member 252 intoopposing lateral sides. Core member 252 further includes a first lateralportion 810 associated with a first lateral side and a second lateralportion 812 associated with a second lateral side.

As seen in FIG. 8, curved engaging surface 262 includes a curvedboundary 820 that lies within coronal plane 704. Curved boundary 820 hasa substantially constant curvature. Specifically, curved boundary 820has a substantially constant arc radius 830. This constant arc radiusfacilitates symmetric lateral bending on both sides of the body.

In some embodiments, arc radius 830 may be substantially smaller thanfirst arc radius 730 and second arc radius 732 (see FIG. 7). In someembodiments, for example, the ratio of arc radius 830 to second arcradius 732 may be approximately 1:2. In some embodiments, arc radius 830may have a value approximately in the range between 2 and 4 centimeters.

By using a core member with a curved engaging surface that has differentarc radii in different regions, the implant described herein providesfor natural motion between adjacent vertebra. In particular, theembodiments described herein allow the top endplate to tilt and slideforwards along the core member during forward flexion. Additionally, theembodiments help limit the tilting and rearward motion of the topendplate along the core member during rearward extension.

In embodiments where the width of a core member varies between theposterior and anterior ends, the arc radius of the outer boundary of thecurved engaging surface could vary. However, in other embodiments, thearc radius of the outer boundary of the curved engaging surface could besubstantially constant from the posterior end to the lateral end.

FIG. 9 is a schematic top down view of core member 252 according to oneembodiment. As seen in FIG. 9, the width of core member 252 may varybetween posterior portion 710 and anterior portion 712. For example,posterior portion 710 may have a maximum width 902 that tapers quicklytowards a posterior edge 904. Moreover, the width of core member 252tapers from maximum width 902 in the direction of anterior portion 712.For reference, an intermediate width 906 is shown for a portion ofanterior portion 712. Here, intermediate width 906 is substantially lessthan maximum width 902. This tapered geometry may help control bendingin the lateral direction and allow for some twisting between core member252 and first endplate 220 (see FIG. 2). In other embodiments, however,core member 252 could have an approximately constant width.

The present embodiments provide an ADR device that facilitates bothtranslation and rotation of the upper endplate relative to the lowerendplate without requiring the use of a core that can slide relative toeither of the endplates. Instead, the curvature of the core member(including posterior and anterior ends with different arc radii) allowthe center axis of rotation of the upper endplate to translate along thecore member. This translation is depicted schematically in, for example,FIGS. 10-13 below.

FIG. 10 is a schematic view of a patient 1000 who has a spinal implant.Adjacent to patient 1000 is an enlarged schematic view of spinal ADRdevice 1001 disposed between an upper vertebra 1030 and a lower vertebra1032. Spinal ADR device 1001 further includes a first endplate 1002, asecond endplate 1004, a core member 1006 and a matrix member 1008.

In FIG. 10, patient 1000 is standing in a neutral position. In thisposition, first endplate 1002 is disposed in neutral position with acentral axis 1003 of first endplate 1002 aligned with a reference axis1040. In this position, core member 1006 and recess 1010 of firstendplate 1002 are in contact along a first contact area 1050. Tofacilitate motion of first endplate 1002 along the curved engagingsurface 1060 of core member 1006, recess 1010 may be shaped toaccommodate the smallest area of curvature on core member 1006. Toaccomplish this, the arc radius associated with recess 1010 may matchthat of the arc radius of the anterior portion 1020 of core member 1006.In the neutral position, therefore, the larger curvature of posteriorportion 1022 may prevent contact between curved engaging surface 1060and recess 1010 at posterior portion 1022.

In FIG. 11, patient 1000 tilts his head backwards, causing adjacentvertebrae in the cervical spine to undergo extension. Referring to FIG.11, as the cervical spine undergoes extension, first endplate 1002 ispushed backward and down the curved engaging surface 1060 untilposterior portion 1022 of first endplate 1002 is engaged by matrixmember 1008. In this position, core member 1006 and recess 1010 are incontact along a second contact area 1150.

Though matrix member 1008 may compress somewhat, depending on the amountof force applied by first end plate 1002, matrix member 1008 nonethelessacts to limit the rearward motion of first end plate 1002. Matrix member1008 may also provide some cushioning and shock absorption following theinitial contact between first end plate 1002 and matrix member 1008.

As seen in FIG. 11, during extension, first endplate 1002 may not onlytilt but may be translated in the posterior direction. Here, centralaxis 1030 has been translated in the posterior direction by a distance1110 and has also rotated with respect to reference axis 1040. However,the variable curvature of core member 1006 is configured to allow forsubstantially more translation in the anterior direction than in theposterior direction, as described in further detail below. The result isthat there is significantly less compression between the endplates atposterior portion 1022 during extension as there is between theendplates at anterior portion 1020 during flexion. This has the effectof limiting the relative translation between vertebrae during extensionand, accordingly, how much the cervical spine can extend back.

In FIG. 12, patient 1000 bends his neck forward, causing adjacentvertebrae in the cervical spine to undergo flexion. Referring to FIG.12, as the spine bends, first endplate 1002 is pushed forward and downthe curved engaging surface 1060 until anterior portion 1020 firstendplate 1002 is engaged by matrix member 1008.

In FIG. 12, as patient 1000 continues pressing his neck down until hischin touches his chest, first endplate 1002 may slide further downcurved engaging surface 1060. Additionally, matrix member 1008 may becompressed, further accommodating the forward translation of firstendplate 1002 and decreasing the relative distance between the top offirst endplate 1002 and the bottom of second endplate 1004 along theanterior side. In this position, core member 1006 and recess 1010 are incontact along a third contact area 1350. Because recess 1010 is sized tofit the larger arc radius of anterior portion 1020, third contact area1350 may be substantially larger than, for example, the contact area1050 when the device is in a neutral position (see FIG. 10).

As seen in FIGS. 12-13, during flexion, first endplate 1002 may not onlytilt but may be translated in the anterior direction. This can be seenclearly by comparing the position of central axis 1003 with referenceaxis 1040 (which represents the initial position of central axis 1003 inthe neutral position). Here, central axis 1030 has been translated inthe anterior direction by a distance 1203 (in FIG. 12) and by a distance1303 (in FIG. 13). Additionally, in both FIGS. 12 and 13, central axis1030 has also rotated with respect to reference axis 1040. In thisexemplary configuration, first endplate 1002 may be configured totranslate by as much as 1 to 3 millimeters in the anterior direction.This allows the adjacent vertebrae to move as they normally would underflexion.

Using a configuration in which the upper endplate can not only tilt butalso translate in the anterior direction relative to the lower endplateallows the adjacent vertebrae to achieve the necessary degree ofcompression so that a patient's neck can move through the entire rangeof normal motion, including putting his chin against his chest.Moreover, using a relatively less rigid matrix member provides shockabsorption during flexion and prevents the upper endplate from cominginto contact with the lower endplate. Thus, the present ADR device actsto provide an increased range of motion in the neck during flexion and amore limited range of motion in the next during extension, accommodatingthe natural motions of the neck.

FIG. 14 is a schematic front view of patient 1000 in a neutral positionwith an enlarged view of device 1001. As seen in FIG. 15, as patient1000 bends his neck laterally, first endplate 1002 travels down curvedengaging surface 1060. During this motion, a central axis 1403 of firstendplate 1002 travels laterally by a distance 1402 compared to areference axis 1404. Because curved engaging surface 1060 has a smallerarc radius along its lateral sides than along its posterior or anteriorsides, the degree of motion is more limited than during extension orflexion. Moreover, because curved engaging surface 1060 has asubstantially constant arc radius, the degree of bending is equal onboth sides.

As already described above, the embodiments not only control the motionof the upper endplate so as to mimic the natural motion of two adjacentvertebrae, but the presence of the softer matrix member substantiallyeliminates contact between the upper and lower endplates. Typically,endplates must be constructed from high strength materials such asmetals that can withstand large contact forces as the endplates collideduring flexion and/or extension. Since the current design eliminatescontact between the endplates, the endplates can be manufactured fromnon-metallic materials. For example, in some embodiments, the endplatescould be manufactured from plastic materials and/or ceramic materialsthat provide better biocompatibility with vertebra, improved wearcharacteristics and different degrees of strength compared to metalmaterials. Moreover, the use of non-metallic materials in the ADR devicemay substantially reduce or eliminate the metallic artifacts that mayarise in MRIs when metallic prosthetics are present in the spine.

In different embodiments, the size and/or relative position of a dome onan upper endplate can vary. For example, FIGS. 16-18 depict threedifferent embodiments in which the dome on the upper endplate varies insize and/or position. Different dome shapes and/or positions can beselected during surgery to facilitate better connection with the uppervertebra. Specifically, the dome may be better aligned and may fitbetter with the natural shape of the inferior side of the vertebra.

In FIG. 16, upper endplate 1602 is configured with a dome 1604 that issubstantially centered along the endplate. Such a configuration may bemost useful for engaging the inferior side 1610 of a vertebra 1612 witha centrally located concave region 1614. In FIG. 17, upper endplate 1702is configured with a dome 1704 that is disposed closer to posterior end1706 of upper endplate 1702. Such a configuration may be most useful forengaging the inferior side 1710 of a vertebra 1712 with a posteriorlylocated concave region 1714. In FIG. 18, upper endplate 1802 isconfigured with a substantially flatter dome 1804 compared to dome 1604and dome 1704. For example, dome 1804 may have a height 1820 compared toa substantially larger height 1620 for dome 1604. Such a configurationmay be most useful for engaging the inferior side 1810 of a vertebra1812 with a relatively shallow concave area 1814.

Embodiments can include additional provisions to prevent contact betweenopposing endplates in an ADR device. In some embodiments, a matrixmember can be shaped to extend radially outward over the lower endplateso that no portion of the lower endplate is exposed on a superior side.

FIG. 19 depicts an alternative embodiment of part of an ADR device 1900that includes a core member 1902, a matrix member 1904, an upperendplate 1908 and a lower endplate 1910. As seen in FIG. 19, matrixmember 1904 includes a peripheral portion 1920 that extends over aperipheral lip 1912 of lower endplate 1910. Using this configuration,matrix member 1904 completely covers the superior side of lower endplate1910 and prevents any contact between lower endplate 1910 and upperendplate 1908.

The exemplary ADR device can be implanted into a patient's spine using asuitable surgical method. In some embodiments, after removing some orall of the tissue between adjacent vertebrae, a surgeon may firstimplant the upper and lower endplates. For example, the surgeon mayplace the lower endplate against one vertebra. In cases where the lowerendplate includes teeth or similar provisions, the surgeon may applypressure and/or tap the endplate to that the teeth engage the vertebraand prevent the lower endplate from moving with respect to the vertebra.The upper endplate may also be placed against the opposing vertebra.Specifically, an upper endplate with a suitable dome for engaging aconcave area in the vertebra can be selected. As described above, asurgeon could choose from at least endplates with domes that are eithercentrally or posteriorly located. Also, a surgeon could choose from atleast endplates with domes that have different overall heights. Once asuitable endplate has been selected, the endplate can be inserted andplaced against the vertebra so that the dome engages the concave area ofvertebra.

Once the endplates have been positioned and/or fixed in place, a surgeonmay insert a core assembly comprising both a core member and a matrixmember. To insert the core assembly, the surgeon can insert the matrixmember into a corresponding recess of the lower endplate whilesimultaneously inserting a portion of the core member into acorresponding recess of the upper endplate. Once the core member hasbeen inserted, the compressive forces of the spine may act to keep thematrix member fixed in the recess of the lower endplate and the coremember engaged with the recess of the upper endplate.

While various embodiments have been described, the description isintended to be exemplary, rather than limiting, and it will be apparentto those of ordinary skill in the art that many more embodiments andimplementations are possible that are within the scope of theembodiments. Although many possible combinations of features are shownin the accompanying figures and discussed in this detailed description,many other combinations of the disclosed features are possible. Anyfeature of any embodiment may be used in combination with, orsubstituted for, any other feature or element in any other embodimentunless specifically restricted. Therefore, it will be understood thatany of the features shown and/or discussed in the present disclosure maybe implemented together in any suitable combination. Accordingly, theembodiments are not to be restricted except in light of the attachedclaims and their equivalents. Also, various modifications and changesmay be made within the scope of the attached claims.

What is claimed is:
 1. An artificial disc replacement device,comprising: a first endplate configured to be disposed against a firstvertebrae in a spine and a second endplate configured to be disposedagainst a second vertebrae in the spine; a core assembly comprising acore member and a matrix member, wherein the core member includes a baseportion and a curved engaging surface, and wherein the base portion ofthe core member is embedded in the matrix member; wherein the firstendplate includes a first recess for receiving the curved engagingportion of the core member; and wherein the second endplate includes asecond recess for receiving the matrix member.
 2. The artificial discreplacement device according to claim 1, wherein: the core memberincludes a sagittal plane that separates the core member into a firstlateral side and a second lateral side; the core member also including acoronal plane that separates the core member into an anterior side and aposterior side; wherein the curved engaging surface includes a curvedboundary that extends within a first plane that is parallel with thesagittal plane; wherein the curved boundary has a first arc radius alongan anterior portion disposed on the anterior side of the core member;and wherein the curved boundary has a second arc radius along aposterior portion disposed on the posterior side of the curved boundary,wherein the second arc radius is substantially different from the firstarc radius.
 3. The artificial disc replacement device according to claim1, wherein the matrix member is substantially less rigid than the coremember.
 4. The artificial disc replacement device according to claim 1,wherein the matrix member is substantially more compressible than thecore member.
 5. The artificial disc replacement device according toclaim 1, wherein the matrix member is comprised of a material includinga gel.
 6. The artificial disc replacement device according to claim 1,wherein the core member is comprised of a material including polyetherether ketone.
 7. An artificial disc replacement device, comprising: afirst endplate configured to be disposed against a first vertebrae in aspine and a second endplate configured to be disposed against a secondvertebrae in the spine; a core member disposed between the firstendplate and the second endplate, the core member having a curvedengaging surface for engaging the first endplate; the core memberincluding a sagittal plane that separates the core member into a firstlateral side and a second lateral side; the core member also including acoronal plane that separates the core member into an anterior side and aposterior side; wherein the curved engaging surface includes a curvedboundary that extends within a first plane that is parallel with thesagittal plane; wherein the curved boundary has a first arc radius alongan anterior portion disposed on the anterior side of the core member;and wherein the curved boundary has a second arc radius along aposterior portion disposed on the posterior side of the curved boundary,wherein the second arc radius is substantially different from the firstarc radius.
 8. The artificial disc replacement device according to claim7, wherein the first arc radius has a value approximately in a rangebetween 4 and 5 centimeters.
 9. The artificial disc replacement deviceaccording to claim 7, wherein the second arc radius has a valueapproximately in a range between 5 and 7 centimeters.
 10. The artificialdisc replacement device according to claim 7, wherein the curvedengaging surface includes a second curved boundary that extends within asecond plane that is parallel with the coronal plane; wherein the secondcurved boundary has a third arc radius that is substantially constant.11. The artificial disc replacement device according to claim 10,wherein the third arc radius is substantially smaller than the first arcradius.
 12. The artificial disc replacement device according to claim11, wherein the third arc radius has a value approximately in the rangebetween 2 and 4 centimeters.
 13. The artificial disc replacement deviceaccording to claim 7, wherein the matrix member is substantially morecompressible than the core member.
 14. An artificial disc replacementdevice, comprising: a first endplate configured to be disposed against afirst vertebrae in a spine and a second endplate configured to bedisposed against a second vertebrae in the spine; a core member disposedbetween the first endplate and the second endplate; a matrix memberdisposed between the first endplate and the second endplate, the matrixmember being in continuous contact with the second endplate; wherein thefirst endplate includes a recess and wherein the core member includes acurved engaging surface in continuous contact with the recess, andwherein the first endplate can move along the curved engaging surface;wherein the matrix member is disposed between the core member and thesecond endplate and prevents contact between the core member and thesecond endplate; wherein the matrix member is positioned to prevent thefirst endplate from contacting the second endplate as the first endplatemoves along the core member.
 15. The artificial disc replacement deviceaccording to claim 14, wherein the second endplate includes a secondrecess that receives the matrix member.
 16. The artificial discreplacement device according to claim 14, wherein a superior surface ofthe matrix member is raised above a superior surface of the secondendplate.
 17. The artificial disc replacement device according to claim14, wherein the second endplate includes a lip that surrounds the secondrecess and wherein the matrix member includes a peripheral portion thatcovers the lip.
 18. The artificial disc replacement device according toclaim 14, wherein the matrix member is substantially more compressiblethan the core member.
 19. The artificial disc replacement deviceaccording to claim 14, wherein a base portion of the core member isembedded within the matrix member.
 20. The artificial disc replacementdevice according to claim 14, wherein: the core member includes asagittal plane that separates the core member into a first lateral sideand a second lateral side; the core member also including a coronalplane that separates the core member into an anterior side and aposterior side; wherein the curved engaging surface includes a curvedboundary that extends within a first plane that is parallel with thesagittal plane; wherein the curved boundary has a first arc radius alongan anterior portion disposed on the anterior side of the core member;and wherein the curved boundary has a second arc radius along aposterior portion disposed on the posterior side of the curved boundary,wherein the second arc radius is substantially different from the firstarc radius.